System and method for the mitigation of paraffin wax deposition from crude oil by using ultrasonic waves

ABSTRACT

A method for mitigating the deposition of wax on production tubing walls. The method comprises positioning at least one ultrasonic frequency generating device adjacent the production tubing walls and producing at least one ultrasonic frequency thereby disintegrating the wax and inhibiting the wax from attaching to the production tubing walls. A system for mitigating the deposition of wax on production tubing walls is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority of PCT Patent ApplicationNo. PCT/US2003/028834, filed Sep. 12, 2003, and U.S. ProvisionalApplication No. 60/410,472, filed Sep. 13, 2002.

The present application is a continuation of pending provisional patentapplication Ser. No. 60/410,472, filed on Sep. 13, 2002, entitled“System and Method for the Mitigation of Paraffin Wax Deposition FromCrude Oil By Using Ultrasonic Waves”.

CONTRACTUAL ORIGIN OF INVENTION

This invention was made with U.S. Government support under Contract No.DE-FC02-91ER75680 awarded by the U.S. Department of Energy. The U.S.Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to system and method for the mitigationof paraffin was deposition from crude oil and, more particularly, theinvention relates to a system and method for the mitigation of paraffinwax deposition from crude oil by using ultrasonic waves.

2. Description of the Prior Art

Wax deposition from crude oil is an enormously expensive problem for oilproducers around the world. In the field, the production tubing is oftenplugged by paraffin wax which deposits on the walls of the productiontubing and surface flow equipment. The deposition of the paraffin leadsto a fall in the production rates of the oil from that well.

The deposition of the paraffin waxes from the reservoir fluid occurswhen the temperature and pressure move below the cloud point of thefluid. The paraffin deposits start off as a thin film and slowlydeposits in the form of crystalline solids, which collects on theinterior of the tubing and flow-lines and slowly chokes off theproduction.

Basically, paraffin deposits are carbonaceous material, which is notsoluble or dispersible by the crude oil under the prevailing conditions.Paraffins are composed primarily of alkanes with formulas C₁₈H₃₈ toC₇₀H₁₇₂. These are straight chained and branch chained compounds, andare generally inert and resistant to attack by acids, bases, andoxidizing agents. Previous research has shown that n-paraffins are moreresponsible for this problem. The formation of the deposit depends onthe cloud point, an available surface and or loss of gas or light endsdue to a drop in pressure. The precipitation is not uniform; it haspeaks at certain points in the tubing and less deposition at otherplaces.

The cloud point temperature is the key factor controlling the paraffinwax deposition. Paraffinic hydrocarbon liquids form a paraffin or waxsolid phase when the temperature falls below the cloud point, or WaxAppearance Temperature (WAT), of the oil. As the oil flows up thewell-bore, its pressure drops causing solution gas to liberate. Thissolution gas which is liberated acts to some degree as a solvent forwaxes. Therefore, the loss of gas increases the cloud point temperaturecausing more precipitation and also makes the oil more viscous.

Also, as the oil moves upward, it cools since the ground temperature isless than the reservoir temperature. There is a temperature gradient atthe wall and when the oil temperature reaches the cloud point theprecipitation starts. This precipitation occurs even if the bulk oiltemperature is more than the cloud point temperature, because it is thetemperature of the oil at the wall, which plays the most important rolein the precipitation of wax. The wax deposition problem is moreprevalent in low flow rate wells because of the high residence time ofoil in the well-bore. The increased flow time leads to more heat loss,which results in lowering of oil temperature and leads to waxprecipitation and deposition. Well-bore studies have shown that thetemperature profile in the well-bore is a strong function of theflow-rate. The paraffin wax problem is an example of fluid/solidequilibrium, which is described as a solution of higher molecular weighthydrocarbons in low molecular weight hydrocarbons which act as solvents.

SUMMARY

The present invention is a method for mitigating the deposition of waxon production tubing walls. The method comprises positioning at leastone ultrasonic frequency generating device adjacent the productiontubing walls and producing at least one ultrasonic frequency therebydisintegrating the wax and inhibiting the wax from attaching to theproduction tubing walls.

In addition, the present invention includes a system for mitigating thedeposition of wax on production tubing walls. The system comprises atleast one ultrasonic frequency generating device adjacent the productiontubing walls and at least one ultrasonic frequency generated by thegenerating device thereby disintegrating the wax and inhibiting the waxfrom attaching to the production tubing walls.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating the system and method formitigation of paraffin wax deposition from crude oil using ultrasonicwaves, constructed in accordance with the present invention;

FIG. 2 is a schematic drawing illustrating the system and method formitigation of paraffin wax deposition from crude oil using ultrasonicwaves, constructed in accordance with the present invention, with thetube in the horizontal orientation;

FIG. 3 is a schematic drawing illustrating the system and method formitigation of paraffin wax deposition from crude oil using ultrasonicwaves, constructed in accordance with the present invention, with thetube in the vertical orientation; and

FIG. 4 is a schematic drawing illustrating an experimental setup of thesystem and method for mitigation of paraffin wax deposition from crudeoil using ultrasonic waves, constructed in accordance with the presentinvention, with an ultrasonic water bath connected to a water coolerpump combination used to circulate water at a fixed temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As illustrated in FIG. 1, the present invention is a system and method,indicated generally at 10, for mitigating the deposition of wax onproduction tubing 12 accumulated from crude oil during production by theuse of ultrasonic waves. The system and method of the present inventionuses ultrasonic waves to disintegrate the wax and inhibit the wax fromattaching to the walls.

The ultrasonic waves or frequencies are generated by at least one deviceor sonde 14 attached to the outside of the production tubing 12 atstrategic locations along its length. While three particular frequencieshave been identified as the optimal frequencies of operation, these areonly a guide for selection of the desirable frequencies of operation. Ina preferred embodiment, the high frequency is approximately five hundred(500) KHz and the low frequency is about ten (10) KHz.

The first frequency is the characteristic frequency of the productiontubing, designated optimal frequency one (OF1). Using the firstfrequency, the ultrasonic waves set the production tubing 12 vibratingthereby inhibiting the wax from depositing on the wall. The secondfrequency (optimal frequency two (OF2)) is the frequency that breaks thewax up into smaller particles by breaking the bonds which cause the waxmolecules to adhere together. The third frequency (optimal frequencythree (OF3)) actually breaks the bonds of the wax molecules so that thelong chained alkanes are broken down into smaller molecules. Thesesmaller molecules will be more soluble in the oil and so will notprecipitate out as wax. Consequently the ultrasonic wave generator 14will be broadcasting at all or any of the three frequencies depending onwhich of the frequencies are not having the desired effect.

In practice, however, these three frequencies would only be a guide forselection of the desirable frequencies of operation. The presentinvention includes a variable frequency device 16 for determining theoptimal frequencies in the range around the three theoretical optimalfrequencies. The ultrasonic broadcast device 14 generates all threefrequencies, once they have been identified by the variable frequencydevice 16.

The three frequencies would have three separate effects. As brieflydescribed above, the OF1 sets the production tubing walls 12 vibratingand hence, inhibits wax molecules from depositing on the walls. Instead,the wax molecules remain entrained in the flowing oil and are carriedaway. The OF2 inhibits the precipitated wax molecules from adheringtogether and from adhering to the walls. The OF3 breaks theunprecipitated long chain wax molecules into smaller molecules and makesthe wax molecules more soluble in the oil thereby lowering the cloudpoint temperature and allowing the molecules to remain in solution. Thecombination of these three effects greatly reduces the wax deposition sothat it is more manageable and removal is required far less frequently.

As illustrated in FIGS. 2 and 3, a paraffin deposition flow system 20has been constructed to simulate the deposition of paraffin in thewells. The flow system 20 consists of two concentric tubes with afacility to measure the pressure drop between the ends of the innertube, called the test section. The crude oil used to conduct theexperiments is stored in a reservoir having a capacity of ten gallons.The crude can be pumped into the test section and back into thereservoir. The flow rate is adjusted using a flow meter and a bypassvalve. An inclined manometer is used to measure the pressure drop acrossthe section. The pressure drop is used to determine the pipe diameterand hence the thickness of the wax deposition. The manometer wasinclined at an angle of thirty-five (35°) degrees to the horizontal andthe manometric fluid is water. A facility to monitor the temperature inthe test section and in the reservoir is also provided. A blower wasrequired to keep the pump from over heating as the experiments are runfor long periods. A water bath attached to a refrigeration unit is usedto provide cooling for the walls of the inner tube.

Water is pumped into the outer annulus and then back into the water bathmaintaining the walls of the test section at the required temperaturethroughout the experiment. At the start of each experiment, themanometer is checked to ensure zero reading and the flow rate adjustedusing pump speed and a bypass valve. Manometer readings are noted atregular intervals until the end of the experiment. At the end ofexperiment, all the pumps and coolers are switched off and test sectionis dissembled. Paraffin that is deposited in the test section is removedusing scrapers and the amount of paraffin measured using a measuringjar. The ultrasonic frequency generating equipment is attached to theoutside of the tube carrying the flowing oil.

The static experimental setup to study the effect of ultrasonic waves onwax deposition is shown in FIG. 4. It consists of an ultrasonic waterbath that was connected to a water cooler pump combination that was usedto circulate the water at some fixed temperature throughout the durationof the experiment.

The foregoing exemplary descriptions and the illustrative preferredembodiments of the present invention have been explained in the drawingsand described in detail, with varying modifications and alternativeembodiments being taught. While the invention has been so shown,described and illustrated, it should be understood by those skilled inthe art that equivalent changes in form and detail may be made thereinwithout departing from the true spirit and scope of the invention, andthat the scope of the present invention is to be limited only to theclaims except as precluded by the prior art. Moreover, the invention asdisclosed herein, may be suitably practiced in the absence of thespecific elements which are disclosed herein.

1. A method for mitigating the deposition of wax on production tubingwalls within a bore hole, the method comprising: positioning at leastone ultrasonic frequency generating device adjacent the productiontubing walls; positioning a variable frequency device outside the borehole; and producing at least one ultrasonic frequency therebydisintegrating the wax and inhibiting the wax from attaching to theproduction tubing walls.
 2. The method of claim 1 and furthercomprising: producing three predetermined frequencies, the frequenciesbeing a first frequency, a second frequency, and a third frequency. 3.The method of claim 2 wherein the three frequencies range betweenapproximately ten (10) KHz and approximately two thousand (2000) KHz. 4.The method of claim 2 and further comprising: producing the firstfrequency; vibrating the production tubing; and inhibiting the wax fromdepositing on the production tubing walls.
 5. The method of claim 2 andfurther comprising: producing the second frequency; and breaking thebonds adhering the wax molecules together thereby disintegrating the waxinto particles.
 6. The method of claim 2 and further comprising:producing the third frequency; reducing the long chained alkanes of thewax molecules thereby reducing the wax into smaller molecules.
 7. Themethod of claim 2 and further comprising: generating all threepredetermined frequencies simultaneously.
 8. The method of claim 1 andfurther comprising: determining the optimal frequencies with thevariable frequency device.
 9. A system for mitigating the deposition ofwax on production tubing walls within a bore hole, the systemcomprising: at least one ultrasonic frequency generating device adjacentthe production tubing walls; a variable frequency device positionedoutside the bore hole; and at least one ultrasonic frequency generatedby the generating device thereby disintegrating the wax and inhibitingthe wax from attaching to the production tubing walls.
 10. The system ofclaim 9 wherein three predetermined frequencies are generated.
 11. Thesystem of claim 10 wherein the three frequencies range betweenapproximately ten (10) KHz and approximately two thousand (2000) KHz.12. The system of claim 10 wherein the first frequency is approximatelyequal to the characteristic frequency of the production tubing therebyvibrating the production tubing and inhibiting the wax from depositingon the production tubing walls.
 13. The system of claim 10 wherein thesecond frequency has a frequency sufficient to disintegrate the wax intoparticles by breaking the bonds which cause the wax molecules to adheretogether.
 14. The system of claim 10 wherein the third frequency has afrequency sufficient to break the bonds of the wax molecules so that thelong chained alkanes are broken down into smaller molecules.
 15. Thesystem of claim 10 and further comprising: generating all threepredetermined frequencies simultaneously.
 16. The system of claim 9 andfurther comprising: the variable frequency device determining theoptimum frequencies.
 17. A method for mitigating the deposition of waxon production tubing walls, the method comprising: positioning at leastone ultrasonic frequency generating device adjacent the productiontubing walls; producing at least one ultrasonic frequency therebydisintegrating the wax and inhibiting the wax from attaching to theproduction tubing walls; and determining the optimal frequencies with avariable frequency device.
 18. The method of claim 17 and furthercomprising: producing three predetermined frequencies, the frequenciesbeing a first frequency, a second frequency, and a third frequency. 19.A system for mitigating the deposition of wax on production tubingwalls, the system comprising: at least one ultrasonic frequencygenerating device adjacent the production tubing walls; at least oneultrasonic frequency generated by the generating device therebydisintegrating the wax and inhibiting the wax from attaching to theproduction tubing walls; and a variable frequency device for determiningthe optimum frequencies.
 20. The system of claim 19 wherein threepredetermined frequencies are generated.